1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, etc. using an electrophotographic system, and particularly relates to convey velocity control of a recording material.
2. Related Background Art
In a conventional image forming apparatus, a recording material such as recording paper, etc. is conveyed onto a recording material conveying path by using a roller, etc. There is a case in which conveying force is simultaneously given to one recording material by different conveying means on upstream and downstream sides of the conveyed recording material in its conveying direction.
In one example of such a conveying form, the image forming apparatus is constructed by a transferring portion for transferring a toner image on a photosensitive drum as an image bearing body to the recording material, and the recording material passing through this transferring portion is conveyed to a fixing nip portion of a fixing portion. A difference in convey velocity is set between the above transferring portion and the above fixing nip portion so as to provide flexure (loop) to the recording material to a certain extent between the above transferring portion and the above fixing nip portion.
There is a system for fixedly setting a preset velocity difference without performing the velocity control as a system for setting such a difference in convey velocity.
The recording material convey velocity of the fixing portion and the recording material convey velocity of the transferring portion are different from each other by thermal expansion of a fixing roller of the fixing portion and an individual difference or a change with the passage of time so that the recording material is tensioned between the above fixing portion and the above transferring portion and an image is deteriorated by this tension. For example, in an image forming apparatus proposed in Japanese Patent Application Laid-Open No. 10-97154, a loop detecting sensor for detecting the loop of the recording material is arranged between the above fixing portion and the above transferring portion as one means for solving the problem of this image deterioration. A control clock period of a stepping motor as a drive motor of the fixing roller is shortened in accordance with detecting results of this loop detecting sensor. Then, a velocity of the drive motor is increased for a constant time and the loop of the recording material is reduced. Thereafter, when a loop amount is reduced, the velocity of the drive motor is returned to its original velocity.
Further, in an image forming apparatus proposed in Japanese Patent Application Laid-Open No. 07-181830, a loop detecting sensor for detecting the loop of the recording material is arranged between the above fixing portion and the above transferring portion. The velocity of a motor for operating a pressurizing roller of the fixing portion is stepwise switched from detecting results of the loop detecting sensor so that the loop amount of the recording material is constantly set.
However, in the above conventional examples, when the recording material is first conveyed with a constant velocity difference without performing the velocity control, a conveying means of a roller, etc. is thermally expanded by e.g., heat of a fixing apparatus and is changed in diameter. Thus, the convey velocity is changed and the velocity difference between front and rear units is increased or reversed. Accordingly, it is considered that this increase in velocity difference, etc. have influence on image quality and conveying performance such as an increase in loop and tension due to a downstream unit.
When the loop of the recording material is detected by the loop detecting sensor such as a photointerrupter, etc., presence/absence of a predetermined amount of loop can be detected. However, for example, it is impossible to perform delicate control in which the tension in the downstream unit is removed while a certain amount of loop is secured at any time, and the recording material is conveyed while rubbing of an image caused by the increase in loop is conversely prevented.
Further, in a color image forming apparatus for transferring plural colors to recording paper, control of the convey velocity is an important problem to provide a color image forming apparatus of high image quality since a change in load during a transferring operation has great influence on a shift in each color, etc.
In particular, in a color LBP of a tandem type for directly transferring four colors of yellow (Y), magenta (M), cyan (C) and black (Bk) to the recording material at any time, the distance between the transfer and the fixation (fixing) is short and there is a state in which the recording material is nipped between plural transferring portions and the fixing means. Therefore, it is important to control the convey velocity between the transfer and the fixation.
Further, when the fixing means is a fixing device of an on-demand system such as an electromagnetic induction system and a film fixing system, the convey velocity of the recording material is greatly dispersed by a kind of the recording material and a continuous sheet passing number in comparison with the conventional fixing device of a heat-pressurizing rubber roller pair having a halogen lamp, etc. within this fixing device so that the loop amount between the transfer and the fixation is greatly changed. Therefore, in the fixing devices of these systems, it is particularly desired to perform delicate control in which the tension in the fixing means is removed while a certain amount of loop is secured at any time, and the recording material is conveyed while rubbing of an image caused by the increase in loop and the shift in each color due to the change in load with respect to the recording material are conversely prevented.
An object of the present invention is to provide an image forming apparatus for preventing an image shift and image rubbing by controlling the flexing amount of a recording material.
Another object of the present invention is to provide an image forming apparatus comprising an image bearing body; a transferring portion for transferring an unfixed image on the image bearing body onto a recording material while conveying the recording material; a fixing portion for fixing the unfixed image onto the recording material while conveying the recording material having the unfixed image transferred in the transferring portion is conveyed; flexure detecting means for detecting flexure of the recording material between the transferring portion and the fixing portion; recording material detecting means for detecting presence/absence of the recording material on a downstream side of the fixing portion with respect to a conveying direction of the recording material; and control means for controlling a convey velocity of the recording material in the fixing portion on the basis of a time from the detection of the recording material detected by the recording material detecting means to the detection of the flexure of the recording material detected by the flexure detecting means.
A still another object of the present invention is to provide an image forming apparatus comprising first conveying means and second conveying means for conveying a recording material; flexure detecting means for detecting flexure of the recording material between the first and second conveying means; recording material detecting means for detecting presence/absence of the recording material on a downstream side of the second conveying means with respect to the conveying direction of the recording material, the first conveying means being arranged on an upstream side of the second conveying means with respect to a conveying direction of the recording material; and control means for controlling a recording material convey velocity of at least one of the first and second conveying means on the basis of a time from the detection of the recording material detected by the recording material detecting means to the detection of the flexure of the recording material detected by the flexure detecting means.
A still another object of the present invention is to provide an image forming apparatus comprising an image bearing body; a transferring portion for transferring an unfixed image on the image bearing body onto a recording material while conveying the recording material; a fixing portion for fixing the unfixed image onto the recording material while conveying the recording material having the unfixed image transferred in the transferring portion; and a guide member arranged over a width of the recording material in a direction perpendicular to a moving direction of the recording material and guiding the recording material to the fixing portion; the guide member being movable and flexure of the recording material between the transferring portion and the fixing portion being detected by movement of the guide member.
Further objects of the present invention will become apparent from the following description.